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Boeing’s Starliner Astronauts Await Earth Return Amid Ongoing Testing

The astronauts aboard Boeing’s Starliner spacecraft, which reached the International Space Station (ISS) last month, are currently in a holding pattern regarding their return to Earth. Astronauts Butch Wilmore and Suni Williams have been in orbit for 51 days, extending their mission beyond the initial timeline due to technical issues with the Starliner’s propulsion system.

Challenges facing the spacecraft involve two main issues: overheating of the reaction control thrusters, which resulted in some shutting down during the spacecraft’s approach to the ISS on June 6, and a notable helium leak within the propulsion system. These complications have prompted engineers to conduct in-depth analyses while searching for solutions.

Despite the challenges, NASA and Boeing officials reiterated their intent for the astronauts to return via the Starliner. Recent efforts include testing a thruster at a facility in White Sands, New Mexico, with plans to conduct additional tests of the thrusters in orbit this weekend while still docked at the ISS. Steve Stich, manager of NASA’s commercial crew program, emphasized that the primary goal remains to secure a safe return for Wilmore and Williams aboard the Starliner.

“We’re honing in on the final elements necessary to ensure a safe return, which is our foremost concern right now,” Stich stated.

There has been speculation regarding a potential switch to a SpaceX Crew Dragon for the astronauts’ return. With one Crew Dragon already docked at the ISS and another scheduled for a new crew launch next month, NASA has explored alternative options. However, Stich affirmed that the focus remains on completing the mission with the Starliner.

“Our prime option is to complete the mission,” he noted. “Bringing Butch and Suni back on Starliner makes sense, especially since the design includes a crew cockpit.”

The Starliner launched from Cape Canaveral Space Force Station in Florida on June 5, marking the spacecraft’s first crewed mission. This test flight is pivotal, aiming to establish a reliable operational framework for future rotations of astronauts to and from the ISS.

Upon successful certification of the Starliner, NASA will have two vehicles capable of transporting astronauts to the ISS, complementing SpaceX’s Crew Dragon, which has successfully ferried astronauts since 2020.

Extending the Test Flight for Data and Confidence

NASA has lengthened the Starliner test flight for additional data collection and testing to bolster confidence in the spacecraft’s capabilities for crewed return and to address the thruster problems. The issues have been identified within the service module, which is designed to detach and incinerate upon reentry, while the reusable crew module is intended to land safely with astronauts aboard.

One critical component of this extended testing has involved sequential firings of a thruster taken from an upcoming mission’s hardware. This thruster underwent rigorous stress tests to simulate the operational conditions it will encounter during flight, including various stages of ascent, undocking, and reentry into Earth’s atmosphere.

“We subjected the thruster to more pulses than we typically expect, putting it through numerous stress points,” Stich explained. “Our tests aligned with the behavior we’ve noted in orbit, with several thrusters showing reduced thrust—a significant observation.”

During last month’s maneuvering, the Starliner’s flight computer deactivated five of its 28 reaction control thrusters, built by Aerojet Rocketdyne, due to overheating issues. After their shutdown, four were subsequently restored to operation, but one was deemed unusable.

Detailed inspections of the ground-tested thruster revealed bulging in a Teflon seal located in an oxidizer valve, indicating potential flow restrictions of the nitrogen tetroxide propellant. The thrusters generate approximately 85 pounds of thrust, relying on the nitrous tetroxide and hydrazine fuel combination to function efficiently.

A specialized poppet valve, akin to a tire inflation valve, is designed to manage the flow of nitrogen tetroxide into the thruster. However, due to the heating and vacuum effects generated during thruster firings, the Teflon seal exhibited deformation potential, raising concerns regarding its integrity during upcoming flight phases.

“The ability of this seal to endure until the conclusion of the mission is essential,” Stich acknowledged, highlighting ongoing evaluations of the seal’s durability as engineering teams work to ensure a safe operational framework for the Starliner’s return.

Source
arstechnica.com